Glasses work by adjusting the focal point of light so that the image reaching the retina of the eye is in focus. Opticians cut the lenses to bring the image closer, further away or correct other vision problems, such as double vision.
Magnifying lenses correct farsighted vision; over-the-counter reading glasses often feature these types of lenses. Farsighted individuals often benefit from prescription lenses the doctor fits individually for each eye. Convex lenses correct nearsighted vision. When the individual has astigmatism, the doctor often prescribes cylindrical lenses. The vision lab grinds these lenses in the shape of a pipe cut lengthwise. Turning the axis of the cylinder adjusts the angle of the astigmatism.
The prescription for eyeglasses expresses the magnification factor in diopters, which indicate the amount of bending of the light. Lens prescriptions include a plus or minus factor. This factor indicates the type of lens the patient requires. Older patients often experience presbyopia, which requires two separate prescriptions, one for distance and a second for reading. In this condition, the cornea or lens of the eye is less able to adjust to distance, so the two separate lenses in eyeglasses adjust the vision. Bifocals allow the combination of the two prescriptions into a single set of glasses.